How to duplicate an object along an edge without warping or rotating it
Instead of arraying a plane along the curve, try arraying a simple vertex. Tweak the constant offset in the array modifier to get the desired offset.



Then create the object you want to instance. Set its parent to the "Arrayed vertices" object. Go to the parent object and in the object properties. Set the instancing to "Verts".


Don't forget to add a high enough resolution in the curve properties or else there may be unwanted interpolation.